Network Working Group J. Rosenberg
Request for Comments: 4453 Cisco Systems
Category: Informational G. Camarillo, Ed.
Ericsson
D. Willis
Cisco Systems
April 2006
Requirements for Consent-Based Communicationsin the Session Initiation Protocol (SIP)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The Session Initiation Protocol (SIP) supports communications across
many media types, including real-time audio, video, text, instant
messaging, and presence. In its current form, it allows session
invitations, instant messages, and other requests to be delivered
from one party to another without requiring explicit consent of the
recipient. Without such consent, it is possible for SIP to be used
for malicious purposes, including spam and denial-of-service attacks.
This document identifies a set of requirements for extensions to SIP
that add consent-based communications.
Table of Contents
1. Introduction ....................................................22. Problem Statement ...............................................23. Requirements ....................................................44. Security Considerations .........................................55. References ......................................................65.1. Normative References .......................................65.2. Informational References ...................................6Rosenberg, et al. Informational [Page 1]

RFC 4453 Consent Requirements April 20061. Introduction
The Session Initiation Protocol (SIP) [1] supports communications
across many media types, including real-time audio, video, text,
instant messaging, and presence. This communication is established
by the transmission of various SIP requests (such as INVITE and
MESSAGE [3]) from an initiator to the recipient, with whom
communication is desired. Although a recipient of such a SIP request
can reject the request, and therefore decline the session, a SIP
network will deliver a SIP request to the recipient without their
explicit consent.
Receipt of these requests without explicit consent can cause a number
of problems in SIP networks. These include amplification attacks.
These problems have plagued email. At the time of this writing, most
SIP services are not interconnected, so the incidence of
amplification attacks directed at SIP services is low compared to the
same attacks on email services. The SIPPING working group believes
it is necessary to address these attacks proactively so the attacks
do not become as burdensome as attacks on email have become.
This document elaborates on the problems posed by the current open
model in which SIP was designed, and then goes on to define a set of
requirements for adding a consent framework to SIP.
2. Problem Statement
In SIP networks designed according to the principles of RFC 3261 [1]
and RFC 3263 [2], anyone on the Internet can create and send a SIP
request to any other SIP user, by identifying that user with a SIP
Uniform Resource Identifier (URI). The SIP network will usually
deliver this request to the user identified by that URI. It is
possible, of course, for network services, such as call screening, to
block such messaging from occurring, but this is not widespread and
certainly not a systematic solution to the problem under
consideration here.
Once the SIP request is received by the recipient, the user agent
typically takes some kind of automated action to alert the user about
receipt of the message. For INVITE requests, this usually involves
delivering an audible alert (e.g., "ringing the phone"), or a visual
alert (e.g., creating a screen pop-up window). These indicators
frequently convey the subject of the call and the identity of the
caller. Due to the real-time nature of the session, these alerts are
typically disruptive in nature, so as to get the attention of the
user.
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RFC 4453 Consent Requirements April 2006
For MESSAGE requests, the content of the message is usually rendered
to the user.
SUBSCRIBE [4] requests do not normally get delivered to the user
agents residing on a user's devices. Rather, they are normally
processed by network-based state agents. The watcher information
event package allows a user to find out that such requests were
generated for them, affording the user the opportunity to approve or
deny the request. As a result, SUBSCRIBE processing, and most
notably presence, already has a consent-based operation.
Nevertheless, this already-existing consent mechanism for SIP
subscriptions does not protect network agents against denial-of-
service (DoS) attacks.
A problem that arises when requests can be delivered to user agents
directly, without their consent, is amplification attacks. SIP
proxies provide a convenient relay point for targeting a message to a
particular user or IP address and, in particular, forwarding to a
recipient that is often not directly reachable without usage of the
proxy. Some SIP proxy servers forward a single request to several
instances or contacts for the same user or resource. This process is
called "forking". Another type of SIP server provides the SIP URI-
list service [5], which sends a new copy of the same request to each
recipient in the URI-list. Examples of URI-list services are
subscriptions to resource lists [6], dial-out conference servers [8],
and MESSAGE URI-list services [7]. A SIP URI-list service could be
used as an amplifier, allowing a single SIP request to flood a single
target host or network. For example, a user can create a resource
list with 100 entries, each of which is a URI of the form
"sip:identifier@target-IP", where target-IP is the IP address to
which the attack is to be directed. Sending a single SIP SUBSCRIBE
request to such a list will cause the resource list server to
generate 100 SUBSCRIBE requests, each to the IP address of the
target, which does not even need to be a SIP node.
Note that the target-IP does not need to be the same in all the
URIs in order to attack a single machine. For example, the
target-IP addresses may all belong to the same subnetwork, in
which case the target of the attack would be the access router of
the subnetwork.
In addition to launching DoS attacks, attackers could also use SIP
URI-list servers as amplifiers to deliver spam. For INVITE requests,
this takes the form of typical "telemarketer" calls. A user might
receive a stream of never-ending requests for communications, each of
them disrupting the user and demanding their attention. For MESSAGE
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RFC 4453 Consent Requirements April 2006
requests, the problem is even more severe. The user might receive a
never-ending stream of visual alerts (e.g., screen pop-up windows)
that deliver unwanted, malicious, or otherwise undesired content.
Both amplification attacks related to spam and DoS can be alleviated
by adding a consent-based communications framework to SIP. Such a
framework keeps servers from relaying messages to users without their
consent.
The framework for SIP URI-list services [5] identifies
amplification attacks as a problem in the context of URI-list
services. That framework mandates the use of opt-in lists, which
are a form of consent-based communications. The reader can find
an analysis on how a consent-based framework helps alleviate
spam-related problems in [9].
3. Requirements
The following identify requirements for a solution that provides
consent-based communications in SIP. A relay is defined as any SIP
server, be it a proxy, Back-to-Back User Agent (B2BUA), or some
hybrid, that receives a request and translates the request URI into
one or more next-hop URIs to which it then delivers a request.
REQ 1: The solution must keep relays from delivering a SIP request
to a recipient unless the recipient has explicitly granted
permission to the relay using appropriately authenticated
messages.
REQ 2: The solution shall prevent relays from generating more than
one outbound request in response to an inbound request, unless
permission to do so has been granted by the resource to whom the
outbound request was to be targeted. This requirement avoids the
consent mechanism itself becoming the focus of DoS attacks.
REQ 3: The permissions shall be capable of specifying that messages
from a specific user, identified by a SIP URI that is an Address-
of-Record (AOR), are permitted.
REQ 4: Each recipient AOR must be able to specify permissions
separately for each SIP service that forwards messages to the
recipient. For example, Alice may authorize forwarding to her
from domain A, but not from domain B.
REQ 5: It shall be possible for a user to revoke permissions at any
time.
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RFC 4453 Consent Requirements April 2006
REQ 6: It shall not be required for a user or user agent to store
information in order to be able to revoke permissions that were
previously granted for a relay resource.
REQ 7: The solution shall work in an inter-domain context, without
requiring preestablished relationships between domains.
REQ 8: The solution shall work for all current and future SIP
methods.
REQ 9: The solution shall be applicable to forking proxies.
REQ 10: The solution shall be applicable to URI-list services, such
as resource list servers [5], MESSAGE URI-list services [7], and
conference servers performing dial-out functions [8].
REQ 11: In SIP, URI-lists can be stored on the URI-list server or
provided in a SIP request. The consent framework must work in
both cases.
REQ 12: The solution shall allow anonymous communications, as long
as the recipient is willing to accept anonymous communications.
REQ 13: If the recipient of a request wishes to be anonymous with
respect to the original sender, it must be possible for the
recipient to grant permission for the sender without the original
sender learning the recipient's identity.
REQ 14: The solution shall prevent attacks that seek to undermine
the underlying goal of consent. That is, it should not be
possible to "fool" the system into delivering a request for which
permission was not, in fact, granted.
REQ 15: The solution shall not require the recipient of the
communications to be connected to the network at the time
communications are attempted.
REQ 16: The solution shall not require the sender of a SIP request
to be connected at the time that a recipient provides permission.
REQ 17: The solution should scale to Internet-wide deployment.
4. Security Considerations
Security has been discussed throughout this document.
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RFC 4453 Consent Requirements April 2006
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